The present invention relates to acoustic resonators, and more particularly, to resonators that may be used as filters for electronic circuits.
The need to reduce the cost and size of electronic equipment has led to a continuing need for ever smaller filter elements. Consumer electronics such as cellular telephones and miniature radios place severe limitations on both the size and cost of the components contained therein. Many such devices utilize filters that must be tuned to precise frequencies. Hence, there has been a continuing effort to provide inexpensive, compact filter units.
One class of filters that has the potential for meeting these needs is constructed from thin film bulk acoustic resonators (FBARs). These devices use bulk longitudinal acoustic waves in thin film piezoelectric (PZ) material. In one simple configuration, a layer of PZ material is sandwiched between two metal electrodes. The sandwich structure is preferably suspended in air by a support structure. When electric field is applied between the metal electrodes, the PZ material converts some of the electrical energy into mechanical energy in the form of mechanical waves. The mechanical waves propagate in the same direction as the electric field and reflect off of the electrode/air interface.
At a resonant frequency, the device appears to be an electronic resonator. When two or more resonators (with different resonant frequencies) are electrically connected together, this ensemble acts as a filter. The resonant frequency is the frequency for which the half wavelength of the mechanical waves propagating in the device is equal to the total thickness of the device for a given phase velocity of the mechanical wave in the material. Since the velocity of the mechanical wave is four orders of magnitude smaller than the velocity of light, the resulting resonator can be quite compact. Resonators for applications in the GHz range may be constructed with physical dimensions on the order of less than 100 microns in lateral extent and a few microns in thickness.
In designing and building miniature filters for microwave frequency usage, it is often necessary to provide resonators (for example, FBARs) having slightly different resonant frequencies, typically a few percent apart. Usually, two distinct frequencies suffice; however, more general filter designs may require three or more resonators each having distinct resonant frequencies. A continuing problem of these filters is to precisely offset the resonant frequencies of the resonators and at the same time allow the resonators to be fabricated on a single wafer, or substrate.
It is known that the frequency of the resonator depends inversely on the thickness of the resonator. To produce multiple resonators having offset frequencies, on a single substrate, one possible technique of mass loading the top metal electrode is disclosed in U.S. Pat. No. 5,894,647 issued to Lakin on Apr. 20, 1999. However, there remains a need for alternative techniques for providing individual resonators having different resonant frequencies on the same substrate.
The need is met by the present invention. According to a first aspect of the present invention, a method for fabricating resonators is disclosed. First, a first bottom electrode and a second bottom electrode are fabricated on a substrate. Above the first and the second bottom electrodes, a core piezoelectric (PZ) layer having a first portion above the first bottom electrode and a second portion above the second bottom electrode is fabricated. Next, an additional PZ layer is fabricated above the first portion. Finally, a first top electrode above the additional PZ layer and a second top electrode above the second portion of the PZ layer are fabricated.
According to a second aspect of the present invention, an apparatus having a first resonator and a second resonator fabricated on a substrate is disclosed. The first resonator has a first bottom electrode; a first top electrode; a first piezoelectric (PZ) material sandwiched between the first bottom electrode and the first top electrode, the first PZ material including a core PZ layer and an additional PZ layer. The second resonator has a second bottom electrode; a second top electrode; a second PZ material sandwiched between the second bottom electrode and the second top electrode.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in combination with the accompanying drawings, illustrating by way of example the principles of the invention.